Fiber optic communication systems typically employ wavelength division multiplexing (WDM), which is a technique for using an optical fiber to carry many spectrally separated independent optical channels. In a wavelength domain, the optical channels are centered on separate channel wavelengths which in dense WDM (WDM) systems are typically spaced apart by 25, 50, 100 or 200 GHz. Information content carried by an optical channel is spread over a finite wavelength band, which is typically narrower than the spacing between channels.
Optical channel monitoring is increasingly being used by telecommunications carriers and multi-service operators of fiber optic systems. As the traffic on optical networks increases, monitoring and management of the networks become increasingly important issues. To monitor the network, the spectral characteristics of the composite signal at particular points in the network must be determined and analyzed. This information may then be used to optimize the performance of the network. Optical channel monitoring is particularly important for modern optical networks that use reconfigurable and self-managed fiber-optic networks.
For example, reconfigurable optical add/drop multiplexers (ROADMs) and optical cross connects, which are used to manipulate individual wavelength channels as they are transmitted along the network, require an optical channel monitor. A ROADM allows dynamic and reconfigurable selection of wavelength channels that are to be added or dropped at intermediate nodes along the network. In a ROADM, for instance, an optical channel monitor can provide an inventory of incoming channels as well as an inventory of outgoing channels and to provide channel-power information to variable optical attenuator (VOA) control electronics so that the power of added channels can be equalized with the pass-through channels.
One type of optical channel monitor employs a wavelength selective switch (WSS), which is a type of switch configured to perform optical switching on a per wavelength channel basis, and is typically capable of switching any wavelength channel at an input fiber to any desired output fiber. Thus, a 1×N WSS can switch any wavelength channel of the WDM input signal propagating along the input fiber to any of the N output fibers coupled to the WSS.
U.S. Pat. Appl. Publ. No. 2010/0046944 shows an optical channel monitor that is incorporated in a WSS. This is accomplished by using the functionality of a 1×1 switch that is available in a 1×N WSS. In particular, the output of the 1×1 switch terminates with a photodiode. In this way, the power of any individual channel can be measured.
While the use of a 1×1 WSS to form an OCM is useful when the optical switching technology is sufficiently fast, this technique is not suitable when used with switches that do not have relatively fast response times. In particular, the optical switching time, the photodiode settling time and the number of channels being monitored determine the OCM loop speed, i.e., the time needed to monitor each channel one time. For many applications OCM loop speeds of less than 1 second, and ideally less than 0.1 second, are desired. Accordingly, the switch and photodiode settling times need to be sufficiently fast to interrogate many channels, which may approach or even exceed 100 in number. To accomplish a 0.2 second loop speed with a photodiode settling time of 1 ms and 100 channels, the optical switching time must also be 1 ms. While this is feasible with some technologies such as digital micro-mirror devices (DMDs) it is not practical with other technologies such as liquid crystal and Liquid Crystal on Silicon (LCoS) technologies.